Power distribution networks are operating at medium voltages between 1 kV and 72 kV and provide electrical power to a plurality of electrical power consumers along a distribution feeder line. The distribution network is protected by a protection system with various protection devices and adapted to disconnect parts of the distribution network from the rest of the distribution network in case of a fault. While the conventional power distribution grid is a passive grid without local energy sources and fully supplied by a transmission grid, more recent focus on renewable energy including photovoltaic systems, small scale hydroelectric power generators, wind turbines and/or gas turbines using biogas, has seen small and medium size Distributed Energy Resources (DER) appear in the distribution grid. With a large amount of controllable and coordinated DER a part of the distribution grid can intentionally disconnect from the transmission grid in emergency cases and continue operation in an islanded mode.
Most electrical energy generation is done by converting energy sources into rotating kinetic energy, and then converting the latter into electrical energy via a synchronous generator. Renewable energy sources such wind and solar power include a final conversion into electrical energy via a Power Electronics (PE) interface including converters or inverters based on power electronic semiconductor components. In addition, energy storage systems such as a flywheel, battery, super conductor or super capacitor energy storage systems also typically use a PE interface as the final conversion method.
In case of a short circuit in a power system a large amount of fault current is produced, which in turn is used to trip protection devices such as circuit breakers or protection relays, and/or blow single-use fuses by melting a fusing conductor inside the fuse. The fault current capability and fault current characteristic of a PE interface may be very different to that of a synchronous generator, to the point that some protection devices used in power systems will not operate as intended. For example a fuse will blow in a short time when subject to fault current sourced from a synchronous generator, but will take too long to blow when the fault current source is a PE interface. Besides increasing, at the expense of additional cost and space, a rating or size of an inverter of the PE interface, conventional solutions to this problem include the following approaches.
i) The settings on protection devices are changed to more sensitive values. When using this solution a different setting will be required for operation with just the PE interfaced energy sources and for operation with a synchronous generator. A third setting may be required for multiple synchronous generators being used since the fault current then will be larger. Yet a fourth setting may be required to avoid false trips when performing cold-load pickup or black starting.
ii) In the case when fuses are used as protection devices, no adaptable settings are available, but the rating or type of fuse may have to be changed to ensure satisfactory operation when subject to fault current from a PE interface. A potential problem with this solution is that careful selection of the fuse size and type is required to make sure discrimination of downstream protection is maintained, and to prevent a downstream fault from blowing the fuse instead of tripping a circuit breaker in the vicinity of the fault. Furthermore a fuse rating and type suitable for the fault current from the PE interface may falsely blow during cold-load pickup.
iii) A synchronous compensator is operated online and providing fault current similar to a synchronous generator. A synchronous compensator uses a synchronous machine to provide voltage control or correct power factor in a power system and has the additional effect of being able to provide fault current similar to a synchronous generator. A drawback of this method is the additional cost and space and loss and operating cost of the synchronous compensator.
iv) A diesel generator is operated online with the prime mover disconnected from the synchronous generator, together with a flywheel or other inertia added to the synchronous generator resulting in a so-called Diesel Uninterruptable Power Supply or D-UPS. In addition to the inertia and UPS capabilities, the synchronous generator provides additional fault current. A drawback of this method is the additional investment cost, space requirement, operating loss and cost of the conventional/diesel generator or D-UPS.
An Induction Machine (IM) is an asynchronous machine operable as an induction motor, as an induction generator, or both. The IM is a type of alternating current (AC) electrical machine that uses the principles of electromagnetic induction to generate the AC currents in the rotor windings. Accordingly, in generator operation, a prime mover from a turbine or engine drives the rotor above the synchronous speed with a negative slip, while in motor operation the shaft provides torque to the load and the rotor operates below synchronous speed with a positive slip.